The present invention relates to a method and apparatus for facilitating mobile data communication, such as high speed data. The invention is specifically related to a new arrangement for assigning carrier tones to a plurality of antennas and a coding technique to provide reliable, high-speed wireless access to mobile users in macrocells.
As more and more people come to rely on wireless communication and as Internet usage becomes more popular as well, it becomes desirable to provide the ability for mobile wireless users to have multimedia access such as to the Internet. However, effective multi-media access requires a high-speed communication capability such as, for example, a bit rate of 1 to 2 Mbps.
It is currently known to provide a wireless data system with high bit rates over a short distance such as in a wireless LAN environment. A co-pending provisional U.S. patent application, entitled CLUSTERED OFDM WITH TRANSMITTER DIVERSITY AND CODING, describes a technique for providing such a high bit rate wireless LAN. In that technique an input data stream is encoded to allow for error/erasure correction in a receiver. Then, a multicarrier (or multitone) signal is formed. For multicarrier, the basic idea is to divide the transmitted bandwidth into many narrow subchannels that are transmitted in parallel. Each subchannel is then modulated at a very low rate to avoid significant intersymbol interference (ISI). The disclosed method employs Orthogonal Frequency Division Multiplexing (OFDM) a multiplexing technique described in for example, xe2x80x9cData Transmission by Frequency Division Multiplexing Using the Discrete Fourier Transformxe2x80x9d by Weinstein et al., IEEE Trans. Commun. Technol. Vol. COM-19, No. 5, October 1971, pp. 628-634 and xe2x80x9cMulticarrier Modulation for Data Transmission: An Idea Whose Time Has Come,xe2x80x9d by Bingham, IEEE Commun. Mag., Vol. 28, No. 5, May 1990, pp. 5-14. In the method disclosed in the provisional application groups of adjacent tones are clustered together and separate clusters are provided to different ones of a plurality of separate independent antennas. A single receive antenna is then used to demodulate the OFDM signal with conventional techniques.
A mobile data system has particular problems which limit the ability to provide high speed multi-media access. The main impairments encountered in a mobile radio environment are delay spread, doppler and path loss as represented by reduced received signal power. Delay spread refers to the fact that because the signal will experience a wireless path that will have different impacts on different frequencies it is likely that the entire signal will not be received at the receiver at the same instant in time. A delay will be introduced. The delay spread in the macrocellular environment could be as large as 40 xcexcsec which could limit the data rate to about 50 Kbaud if no measures are taken to counteract the resulting ISI. In the 2 GHz PCS bands the doppler rate could be as high as 200 Hz (i.e., a mobile unit moving at about 67 mph). Furthermore, the received signal power is inversely related to the data rate such that, for example, at a data rate of 1 Mbaud (approximately 50 times that of a typical voice circuit) there is a shortfall of at least 15 dB in received power compared to cellular voice services and this creates a link budget problem. Thus, without any system modification the coverage and performance of such systems will be severely limited. In fact, in the present wireless systems that cover a wide area with mobile receivers, bit rates of 10 to 20Kbps have been achieved. Therefore, it is desirable to adapt the wireless transmission systems to facilitate high-speed data communications.
The present invention achieves the desired high-speed wireless transmission by modifying the system to correct for the effects of delay spread and path loss. The present invention proposes an asymmetric service: a high-speed down link (for example 1 to 2 Mbps peak data rates, or more) and a lower bit rate lower uplink (for example 50-100 Kbs). This would alleviate the problem of increasing power consumption at the mobile terminal to overcome the 15 dB shortfall in received power. Nonetheless, it should still be sufficient for most applications, such as Web browsing, voice access, e-mail, and interactive computing.
Furthermore, the present invention provides an Orthogonal Frequency Division Multiplexing (OFDM) system that has narrow enough subchannels and sufficient guard period to minimize the effects of delay spreads as large as 40 xcexcsec.
To overcome the 15 dB shortfall in link-budget, the present invention provides transmit antenna diversity and coding across frequencies. In one example the base station has four transmit antennas. Each antenna is assigned to transmit a subset of the total number of tones. A particular subset is composed of a plurality of widely spaced tones covering the entire transmission bandwidth. As a consequence a subset of tones on a second antenna will include tones between those transmitted on the first antenna. Alternatively each subset of tones for a given transmit antenna can include widely spaced clusters of tones, e.g., two or three adjacent tones, which cover the entire transmission bandwidth. Spreading the tones over the transmit antennas randomizes the fading across the OFDM bandwidth.
The coding is also selected to help reduce the link-budget problem. The digital data are encoded using Reed-Solomon (R-S) encoding where symbol words within R-S codewords are created by time-grouping modulation symbols that are consecutive in time. The encoding uses a combination of erasure correction, based on signal strength, and error correction.
When the tone antenna assignment technique and the coding operation are combined the link-budget problem is substantially alleviated.
In alternative embodiments the mobile station may include receive antenna diversity. Also, the assignment of tones to the transmit antennas can be arranged such that the same tone is transmitted simultaneously by two or more antennas. In yet another modification the tones assigned to a given antenna can be changed over time so that the effect of any negative correlation between a given tone and a given transmission path from a transmit antenna to a receive antenna can be minimized.